The GAO has released report titled IMMIGRATION BENEFITS Improvements Needed to Address Backlogs and Ensure Quality of Adjudications, dated November 2005. Of particular interest is an explanation by the GAO of the curious procedure used by the CIS to calculate their "official backlog:"
According to USCIS, the data management systems it currently uses to manage its backlog elimination efforts cannot comprehensively produce data to measure and track the time that all applications have been pending, and therefore the agency cannot readily retrieve information on the number of applications that have been pending for more than 180 days, as specified in the definition of backlog in the Immigration Services and Infrastructure Improvements Act of 2000. Instead, USCIS estimates its backlog based on the number of pending applications in excess of the applications it received during the past 6 months. For example, if the agency had received 100,000 applications for benefits in the most recent 6 months and currently had 120,000 cases awaiting adjudication, it would report a backlog of 20,000 cases. The agency’s rationale for using this proxy is that by consistently completing more applications than are filed each month, the agency should gradually reduce its pending workload of applications to a level at which it can complete all incoming applications within the workload targets established for each application type. Eventually, according to the agency’s backlog elimination plan, as long as USCIS is processing all applications received within the past 6 months (or less, depending on the application type’s workload target) there should be no backlog because those applications awaiting adjudication should be completed before they become part of the backlog count of applications pending longer than 6 months. However, USCIS’s definition of backlog does not guarantee that every applicant requesting a benefit will receive a decision within 6 months of filing.
This is from immigration-information.com
According to USCIS, the data management systems it currently uses to manage its backlog elimination efforts cannot comprehensively produce data to measure and track the time that all applications have been pending, and therefore the agency cannot readily retrieve information on the number of applications that have been pending for more than 180 days, as specified in the definition of backlog in the Immigration Services and Infrastructure Improvements Act of 2000. Instead, USCIS estimates its backlog based on the number of pending applications in excess of the applications it received during the past 6 months. For example, if the agency had received 100,000 applications for benefits in the most recent 6 months and currently had 120,000 cases awaiting adjudication, it would report a backlog of 20,000 cases. The agency’s rationale for using this proxy is that by consistently completing more applications than are filed each month, the agency should gradually reduce its pending workload of applications to a level at which it can complete all incoming applications within the workload targets established for each application type. Eventually, according to the agency’s backlog elimination plan, as long as USCIS is processing all applications received within the past 6 months (or less, depending on the application type’s workload target) there should be no backlog because those applications awaiting adjudication should be completed before they become part of the backlog count of applications pending longer than 6 months. However, USCIS’s definition of backlog does not guarantee that every applicant requesting a benefit will receive a decision within 6 months of filing.
This is from immigration-information.com